1. Field of Invention
This invention relates to improved apparatus and a method of measuring groundwater flow. More specifically, it relates to apparatus and a method of measuring groundwater flow at very low seepage velocities and direction by the introduction of heat into the groundwater and sensing the distribution of the heat with temperature sensors.
2. Prior Art
There are many methods and devices for measuring fluid flow in the prior art. For example, U.S. Pat. No. 3,498,127 discloses a method wherein an energy source, such as a heater, is immersed in a moving fluid and wherein changes in fluid properties, such as temperature, are used to detect and measure flow velocity and direction. U.S. Pat. No. 4,628,743 discloses a device wherein a heat pulse is injected into the flowing stream by a miniature thermistor and the detection of an electronic time derivative of temperature downstream triggers subsequent heat pulses, the pulse frequency being related to flow rate. U.S. Pat. No. 4,916,948 discloses the use of thermal flow sensors, such as thermosensitive resistors, for air flow measurement. U.S. Pat. No. 5,412,983 discloses the measurement of fluid velocity with a single temperature-dependent resistor in a partially-open measurement chamber. All the fluid flow measurement methods and devices are for relative high fluid velocities, much higher than would be encountered in underground water flow. The prior-art methods and devices for measuring fluid velocity are, therefore, unsuitable for underground water monitoring.
U.S. Pat. No. 5,339,694 discloses a monitoring probe for detecting groundwater migration based on electrical conductivity. Salt water is filled into a cylinder having a permeable membrane and a plurality of electrical conductivity sensors on its outer surface. As the salt water diffuses into the groundwater, electrical conductivity around the cylinder is monitored by the sensors and analyzed to determine groundwater flow velocity and direction. U.S. Pat. No. 3,352,154 relates to an apparatus for measuring the motion of gases, specifically air, consisting of a refractory cylinder upon whose surface an electrically conductive ring and a pair of electrically conductive metal strips are affixed. These are made of a metal having a high temperature coefficient of electrical resistance. They are electrically heated and their resistance is used to determine temperature and air flow rate. U.S. Pat. No. 4,547,080 discloses a convective heat flow probe having a plurality of heater pads each with an electric heater and a temperature sensor, for determining heat and fluid flow in the formation surrounding a borehole.
One system of groundwater flow monitoring that has been used in the past comprises digging or drilling vertical wells into the ground water aquifer. The wells typically have an inside diameter from 2 to 6 inches. Smooth-walled plastic casings and screens free of internal obstructions are installed. A heater/temperature probe assembly is then installed such that it is immersed in the groundwater at the depth of interest in the screened section, with wiring connecting the probe assembly with measurement equipment at the surface. A serious drawback of this conventional system, as disclosed in U.S. Pat. No. 4,391,137, is the use of thermistors for temperature measurement. Owing to the extreme nonlinearity of the relationship between temperature and thermistor response, the system has to be corrected for each water temperature, and the probe must be rotated 180.degree. to eliminate sensor bias. As a result, groundwater flow measurements are enormously time consuming with this apparatus.
There is a need, therefore, for improved apparatus and an improved method of measuring and monitoring groundwater flow at extremely low seepage velocities (0.1-10 ft/day). The object of this invention is to provide a system for determining groundwater flow velocity and direction which is more convenient, easier to calibrate, and more accurate than the systems of the prior art.